Method and apparatus for processing a pulverulent or particulate material

ABSTRACT

In a fluid bed apparatus in which a layer of pulverulent or particulate material is fluidized by means of fluidizing gas which is introduced through openings in a bed plate in a first plurality of streams of flows of gas, means are provided for introducing a second plurality of streams of flows of gas at a site or sites of introduction and is directed from said site or sites along the surfaces of at least some of the walls of the housing to form a gas barrier covering at least a substantial part of said wall surfaces and the ceiling of the housing thus reducing detrimental effects, in particular deposition of products, condensation and corrosion, on the surfaces of the walls and ceiling of the housing.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for processing alayer of pulverulent or particulate material which is fluidized on a bedplate of a fluid bed apparatus by means of fluidizing gas which isintroduced through openings in the bed plate in a first plurality ofstreams or flows of gas, said fluid bed apparatus comprising a housinghaving a mainly polygonal shape and comprising a number of walls and aceiling, said method further comprising a reduction of detrimentaleffects, in particular deposition of products, condensation andcorrosion, on the surfaces of said walls and ceiling.

BACKGROUND OF THE INVENTION

In the area of fluid bed technology it is a well-known problem thatcondensation is generated on the walls and the ceiling of the housingdue to the high moisture content of the fluidizing gas after the passagethrough the layer of material to be processed in the fluid bed. It is afurther problem that products may deposit on these walls.

In order to remedy these problems it is known, as eg. described in U.S.Pat. No. 4,492,040 (Niro), to provide the inner side of the housing withheating means, eg. heating panels which are heated in a suitable manner,for example by means of electric current, heated gas or air, or steam orwater, in order to avoid condensation and deposition of products on thewalls and ceiling of the housing.

Furthermore, it is known to direct a so-called bypass gas to the fluidbed, which is mixed with the fluidizing gas in order to obtain a gasmixture having a lower moisture content in order to avoid condensationin the discharge system of the fluid bed apparatus.

However, in fluidization processes involving materials which during thedrying release corrosive vapour, such as eg. s-PVC which releases HCl,the above-mentioned measures may not be sufficient.

SUMMARY OF THE INVENTION

With this background it is an object of the present invention to providea method and apparatus in which the above-mentioned undesirableconsequences of the drying process is largely avoided.

In one aspect of the invention this object is met by a method of theinitially stated kind, which is characterized in that a second pluralityof streams or flows of gas is introduced into said housing at a site orsites of introduction and is directed from said site or sites along thesurfaces of at least some of the walls of the housing to form a gasbarrier covering at least a substantial part of said wall surfaces andthe ceiling of the housing.

By forming a gas barrier which is brought to cover the parts of thehousing which are subjected to the vapour deriving from the materialbeing dried in the dryer, a good protection against contact between themoist and possibly corrosive spent fluidizing gas/product and the wallsand ceiling of the fluid bed housing is obtained, thus impeding theformation of condensation and the deposition of products, andcounteracting the effects of corrosive vapour, such as pitting andstress corrosion and of product degradation.

In another aspect of the invention a fluid bed apparatus is provided,said apparatus comprising a housing having a mainly polygonal shape andcomprising a number of walls and a ceiling, and a bed plate on which alayer of pulverulent or particulate material is fluidized by means offluidizing gas which is introduced through openings in the bed plate ina first plurality of streams or flows of gas, means being provided forreducing detrimental effects, in particular deposition of products,condensation and corrosion, on the surfaces of said walls and ceiling.According to the invention the fluid bed apparatus is characterized inthat means are provided for introducing a second plurality of streams orflows of gas at a site or sites of introduction and is directed fromsaid site or sites along the surfaces of at least some of the walls ofthe housing to form a gas barrier covering at least a substantial partof said wall surfaces and the ceiling of the housing.

Advantageous embodiments of the invention are the subject of thedependent claims.

In the following the invention will be described in further detail bymeans of an embodiment thereof and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are very schematic sectional views of an embodiment of theapparatus according to the invention; and

FIG. 3 is a graphic illustration of a computer-simulated example of thetemperature distribution with application of the method and apparatusaccording to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1 and 2 show cutout sectional views of a first 1 and a second 5section, respectively, of a fluid bed apparatus which is used forprocessing a pulverulent or particulate material. Examples of suchmaterials are dairy products, ceramics, polymers etc. which are eg.dried, cooled or spray granulated in the fluid bed apparatus. The methodand apparatus according to the invention are particularly advantageousin connection with materials containing chlor, such as inorganic saltsor polymers as PVC.

The first section 1 of the apparatus is in the embodiment shown of theback-mix type, in which pulverulent or particulate material is suppliedby distributor means (not shown) to a fluid bed plate 2 on which thematerial forms a layer of material P. The height or thickness of thematerial depends on the type of material being processed and may beregulated by a suitable means such as a damper or an overflow valve.Fluidizing gas is introduced through openings 3 in the bed plate 2 froma plenum chamber 4 in a plurality of streams or flows of gas. The plenumchamber 4 is in a manner known per se supplied with a suitablefluidizing gas such as heated ambient air. The fluid bed plate 2 may eg.be of the type which is described in Applicant's European Patent No. 0474 949. A number of heating panels (not shown) are preferably arrangedabove the bed plate 2 in this section of the fluid bed apparatus.

The second section 5 of the apparatus is of the plug flow type and has abed plate 6 with openings 7, through which fluidizing gas is introducedfrom a plenum chamber 8. The second section 5 comprises a number ofbaffle walls (not shown) providing a serpentine path for the material.The bed plate 6 may be designed as the bed plate 2 of the first section1 or as described in Applicant's European Patent No. 0 430 849.

The first and second sections 1 and 5 have a common housing having amainly polygonal shape, the bed plates 2 and 6 each having asubstantially polygonal or rectangular shape, and a number of wallsextending perpendicularly to the bed plates, of which walls 9 and 10 areshown in FIGS. 1 and 2. At the upper end of the walls, these areconnected to a ceiling (not shown) which may be substantially parallelto the bed plates, thus forming a substantially box-shaped fluid bedapparatus. However, the transition between the walls and the ceiling maybe designed as an area having an oblique or rounded shape. Exhaust means(not shown) are provided at the ceiling for withdrawing spent fluidizinggas, either to the ambience after cleaning of the gas, or the gas may berecycled after going through a series of cleaning, condensing andheating processes.

In order to reduce the detrimental effects, such as deposition ofproducts as such or in the form of fines entrained in the fluidizinggas, condensation and possibly corrosion, of the spent fluidizing gas onthe walls and ceiling of the fluid bed apparatus housing, means 11 and17 are provided in each section 5, 1 for providing a gas barrier on thewalls and ceiling of the housing.

The introduction means 11 of the second section 5 will be described infurther detail below.

Barrier gas is led from the plenum chamber 8 to the introduction means11 through a passage which is formed in the hollow space between thewall 10 and an inner wall 12. Alternative barrier gas sources are ofcourse conceivable. It is thus possible to make use of a separate plenumchamber, external supply means, the plenum chamber 4 of the firstsection 1 etc. At the upper end of the passage between the wall 10 andthe inner wall 12, the gas is passed in a plurality of tubes 13positioned at a distance from each other in the longitudinal directionof the wall 10. The tubes 13 are connected to a profile 14 extending inthe longitudinal direction of the wall 13, ie. perpendicularly to thetubes 13. The profile 14 has on its side facing the wall 10 a plate 14 ain which openings are provided, through which the barrier gas may pass.The plate 14 a may be of any suitable kind, such as a perforated plate,a gill-type plate or comprise baffles, which provides for the desiredvelocity distribution and values.

The barrier gas is thus introduced into the inner space of the housingin a second plurality of streams or flows which are directed along thesurface of the wall 10 and continues to flow along the wall 10 and theceiling of the housing. In the embodiment shown, a protruding flangeportion 14 b on the profile 14 ensures that the barrier gas is directedalong the wall 10. Similar introduction means may be provided onadjoining walls in order to obtain a substantially continuous gasbarrier covering the wall 10, adjoining walls and the ceiling.

Alternatively, the second plurality of flows or streams forming the gasbarrier may be introduced by means of nozzles giving eg. a fan-shapeddistribution of jets. However, this is not as advantageous as theembodiment shown and described, as nozzles impart an unequaldistribution and a higher velocity to the streams or flows of barriergas. The barrier gas may furthermore be introduced through verticalslits at the corners of the housing, ie. at the connection betweenadjoining walls, which entails that the streams or flows of gas aredirected substantially horizontally.

A shielding arrangement in the form of a deflecting plate 15 is securedto the profile 14. The deflecting plate 15 is intended for protectingthe introduction means 11 from spouting material from the fluid bed. Inaddition, the deflecting plate 15 leads the fluidizing gas from the bedin a direction away from the wall 10 so that mixing between the barriergas and fluidizing gas is diminished. An aperture 15 a is provided inthe deflecting plate 15 to allow spouting material which has enteredabove the deflecting plate 15 to return to the fluid bed. Spoutingmaterial which has entered the space between the plate 14 a of theprofile 14 and the wall 10 is allowed to return to the bed through anarrow gap 16 to avoid clogging of the openings in the plate 14 a.

The position of the introduction means, ie. the site of introduction ofthe barrier gas, may be varied according to the character of thematerial being fluidized and the height or thickness of the materiallayer on the bed plate. In the second or plug flow section 5 shown inFIG. 2, the introduction means 11 are positioned well above the layer ofmaterial P on the bed plate 6. As the temperature of the barrier gas isrelatively high and preferably higher than the temperature in the fluidbed, deposition of products and condensation on the part of the wallsituated below the introduction means 11 is reduced by passing thebarrier gas in the hollow space between the wall 10 and the inner wall12. The temperature of the barrier gas may vary but should not, however,be so high so as to cause damage to the material being fluidized.

As shown in FIG. 1, the introduction means 17 in the first or back-mixsection of the fluid bed apparatus is situated at a level only slightlyabove the layer of material P on the bed plate 2. The introduction means17 may eg. be designed as the introduction means 11 of the secondsection 5, but the barrier gas is passed from the plenum chamber 4 tothe introduction means 17 through a tube 18 positioned externally of thewall 9 of the first section 1. As described in the above, it is ofcourse possible to use other sources of gas, and the barrier gas mayalso be used for heating the wall 9 below the site of introduction as inthe second section of the fluid bed apparatus.

The amount of the barrier gas typically lies in the interval 2 to 25% ofthe total amount of gas, ie. the sum of the amounts of fluidizing gasand barrier gas, and the velocity of the barrier gas should besufficiently low so as not to entrain the fluidizing gas, as this wouldreduce the effect of the gas barrier. This velocity which may typicallylie in the range 0.2 to 15 times the velocity of the fluidizing gas, eg.values of the velocity being 0.1 to 5 m/s.

EXAMPLE

A flow of s-PVC is supplied to the back-mix section 1 of the fluid bedapparatus and is dried by means of fluidizing gas having a temperatureof about 70° C. which is introduced through the openings 3 and 7 in thebed plates 2 and 6 of the first (back-mix) and second (plug flow)section, respectively. The temperature of the material on the bed plates2 and 6 is slightly lower, approx. 60° C. The temperatures may bedifferent in different parts of the fluid bed apparatus.

The velocity of the fluidizing gas is approx. 0.35 m/s in the back-mixsection 1 and approx. 0.25 m/s in the plug flow section 5.

Barrier gas is introduced through the introduction means 11 and 17. Theamount of barrier gas is approx. 9% of the total amount of gas.

As may be seen from the graphic illustration of FIG. 3 showing thetemperature distribution in a computer-simulation using conditionscorresponding to the above-mentioned and at a velocity of the barriergas of 2 m/s, the temperature at the walls and the ceiling are increasedin relation to the temperature in the fluid bed.

The invention should not be regarded as being limited to the embodimentdescribed in the above but various modifications of the method andapparatus may be carried out without departing from the scope of theappended claims.

1. A method of processing a layer of pulverulent or particulate materialwhich is fluidized on a bed plate of a fluid bed apparatus by means offluidizing gas which is introduced through openings in the bed plate ina first plurality of streams or flows of gas, said fluid bed apparatuscomprising a housing having a polygonal shape and comprising a number ofwalls and a ceiling, said method further comprising a reduction ofdetrimental effects in particular deposition of products, condensationand corrosion, on the surfaces of said walls and ceiling, wherein asecond plurality of streams or flows of gas is introduced into saidhousing at a site or sites of introduction and is directed from saidsite or sites along the surfaces of at least some of the walls of thehousing to form a gas barrier covering at least a substantial part ofsaid wall surfaces above said bed plate and the ceiling of the housing,the temperature of said second plurality of streams or flows of gasbeing higher than the temperature of the layer of material.
 2. A methodaccording to claim 1, wherein said second plurality of streams or flowsof gas has a first flow component directed upwardly form said site orsites of introduction and/or a second flow component at right angles tosaid first flow component.
 3. A method according to claim 1, whereinsaid site or sites of introduction is/are situated at a level above thelayer of material on the plate.
 4. A method according to claim 1,wherein the velocity of the streams or flows or gas of said secondplurality is sufficiently low so as not to entrain said fluidizing gas.5. A method according to claim 1, wherein the amount of said secondplurality of streams or flows of gas is 2–25% of the total amount ofgas.
 6. A method according to claim 1, wherein said second plurality ofstreams or flows of gas is utilized for heating said wall or walls belowthe site or sited of introduction.
 7. A method according to claim 1, inwhich said a fluid bed apparatus comprises a first fluid bed section ofthe back-mix flow type and a second fluid bed section of the plug flowtype, wherein said second plurality of streams or flows of gas isutilized for heating said wall or walls below the site or sites ofintroduction in at least one of said sections.
 8. A fluid bed apparatuscomprising a housing having a polygonal shape and comprising a number ofwalls and a ceiling, and a bed plate on which a layer of pulverulent orparticulate material is fluidized by means of fluidizing gas which isintroduced through openings in the bed plate in a first plurality ofstreams or flows of gas, means being provided for reducing detrimentaleffects, in particular deposition of products, condensation andcorrosion, on the faces of said walls and ceiling, wherein means areprovided for introducing a second plurality of streams or flows of gasat a site or sites of introduction and is directed from said site orsites along the surfaces of at least some of the walls of the housing toform a gas barrier covering at least a substantial part of said wallsurfaces above said bed plate and the ceiling of the housing, saidintroduction means comprising a profile extending in the longitudinaldirection of the wall or at least one nozzle at each site ofintroduction.
 9. A fluid bed apparatus according to claim 8, whereinsaid introduction means are designed to impart to said second pluralityof streams of flows of gas a first flow component directed upwardly fromthe introduction means and/or a second flow component at right angles tosaid first flow component.
 10. A fluid bed apparatus according to claim8, wherein said introduction means are situated at a level above thelayer of material on the bed plate.
 11. A fluid bed apparatus accordingto claim 8, wherein said introduction means are designed as openings inthe wall.
 12. A fluid bed apparatus according to claim 8, wherein ashield arrangement is provided at the site or sites of introduction fordeflecting spouting material from the fluid bed from the introductionmeans.
 13. A fluid bed apparatus according to claim 12, wherein anaperture is provided in said shielding arrangement for permittingspouting material to return to the fluid bed.
 14. A fluid bed apparatusaccording to claim 8, wherein said wall or walls is/are hollow in partor in full to conduct at least said second plurality of streams or flowsof gas.
 15. A fluid bed apparatus according to claim 14, wherein saidsecond plurality of streams or flows of gas is utilized for heating saidwall or walls.
 16. A fluid bed apparatus according to claim 8,comprising a first fluid bed section of the back-mix flow type and asecond fluid bed section of the plug flow type, wherein said secondplurality of streams or flows of gas is utilized for heating said wallor walls in at least one of said sections.